The use of chelating agents of various types to entrap metal ions useful in magnetic resonance imaging is well known. Generally, the chelating agents contain a substantial number of unshared electron pairs or negatively charged or potentially negatively charged species. Perhaps the simplest among these is ethylenediaminetetraacetic acid (EDTA) commonly used as a water softener. Other chelating agents are diethylene triamine pentaacetic acid (DTPA), tetraazacyclododecanetetraacetic acid (DOTA), and their derivatives. In order to attach these chelating agents to additional moieties so as to permit other agents to be associated with them, the basic chelate nucleus has been derivatized. For example, U.S. Pat. No. 6,221,334 describes coupling folate receptor binding ligands to DOTA-type chelates.
One approach to such derivatization is exemplified in U.S. Pat. Nos. 5,652,361, 5,756,065, and 5,435,990, where the methylene group adjacent one of the carboxyl groups of DOTA is the point of attachment. This creates a chiral center, which enhances the complexity of any further reaction. In some cases, this methylene is coupled to a benzyl or phenyl moiety wherein the phenyl ring is substituted by a reactive group, such as isothiocyanate. The isothiocyanate provides a means for coupling to various additional compounds. As described in these patents, the isothiocyanate group can be used to couple the chelate to a targeting agent such as an antibody or fragment thereof.
Derivatized DOTA molecules are described for example, in U.S. Pat. Nos. 5,358,704; 4,885,363; 5,474,756; 5,674,470; 5,846,519; and 6,143,274, where the point of attachment is at one of the DOTA ring nitrogens. These derivatized DOTA molecules are stated, in these cases, to have the advantage of being neutral in solution. An additional patent that discloses attachment to the ring N of DOTA is U.S. Pat. No. 5,310,535. Other DOTA derivatives are described in U.S. Pat. No. 5,573,752, where one carboxyl is replaced with an amide, further bound to an aromatic system. Self-assembling forms of chelating agents are described in U.S. Pat. No. 6,056,939. (See also, Auffer, et al., Chem. Rev. (1999) 99:2293-2352; Gali, et al., Anticancer Res. (2001) 21:2785-2792; Sherry, et al., Inorg. Chem. (1989) 28:620-622; and Aime, et al., Inorg. Chem. (1992) 31:2422-2428).
There is an extensive literature on delivery vehicle compositions that have been used to administer chelated metals for MRI. Some of these compositions do not contain targeting agents, though others do comprise such agents. For example, U.S. Pat. Nos. 5,690,907; 5,780,010; 5,989,520; 5,958,371; and PCT publication WO 02/060524, the contents of which are incorporated herein by reference, describe emulsions of perfluorocarbon nanoparticles that are coupled to various targeting agents and to desired components, such as MRI imaging agents, radionuclides, and/or bioactive agents. Other compositions that have been used for targeted imaging include those disclosed in PCT publications WO 99/58162; WO 00/35488; WO 00/35887; and WO 00/35492, each of which is incorporated herein by reference.
Magnetic resonance imaging (MRI) has become a useful tool for diagnosis and for research. The current technology relies on detecting the energy emitted when the hydrogen nuclei in the water contained in tissues and body fluids returns to a ground state subsequent to excitation with a radio frequency. Observation of this phenomenon depends on imposing a magnetic field across the area to be observed, so that the distribution of hydrogen nuclear spins is statistically oriented in alignment with the magnetic field, and then imposing an appropriate radio frequency. This results in an excited state in which this statistical alignment is disrupted. The decay of the distribution to the ground state can then be measured as an emission of energy, the pattern of which can be detected as an image.
However, the relaxation rate of the relevant hydrogen nuclei in the above described process is too slow to generate detectable amounts of energy, as a practical matter. To create a detectable signal, the area to be imaged is supplied with a contrast agent, generally a strongly paramagnetic metal, which effectively acts as a catalyst to accelerate the decay, thus permitting sufficient energy. Thus, contrast agents decrease the relaxation time and increase the reciprocal of the relaxation time—i.e., the “relaxivity” of the surrounding hydrogen nuclei.
Two types of relaxation times can be measured. T1 is the time for the magnetic distribution to return to 63% of its original distribution longitudinally with respect to the magnetic field and the relaxivity τ1, is its reciprocal. T2 measures the time wherein 63% of the distribution returns to the ground state transverse to the magnetic field. Its reciprocal is the relaxivity index τ2. In general, the relaxation times and relaxivities will vary with the strength of the magnetic field; this is most pronounced in the case of the longitudinal component.
Contrast agents which are based on chelated paramagnetic metal have been described. For example, U.S. Pat. Nos. 5,512,294 and 6,132,764 describe liposomal particles with metal chelates on their surfaces as MRI contrast agents. U.S. Pat. Nos. 5,064,636 and 5,120,527 describe paramagnetic oil emulsions for MRI in the gastrointestinal tract. U.S. Pat. Nos. 5,614,170 and 5,571,498 describe emulsions that incorporate lipophilic gadolinium chelates, e.g., gadolinium diethylenetriaminepentaacetic acid-bisoleate (Gd-DTPA-BOA) as blood pool contrast agents. U.S. Pat. No. 5,804,164 describes water-soluble, lipophilic agents which comprise particularly designed chelating agents and paramagnetic metals. U.S. Pat. No. 6,010,682 and other members of the same patent family describe lipid soluble chelating contrast agents containing paramagnetic metals which are said to be able to be administered in the form of liposomes, micelles or lipid emulsions. Thus, in general, contrast agents may take the form of paramagnetic metals such as rare earth metals or iron mobilized in a form that permits substantial concentrations of the paramagnetic metal to be delivered to the desired imaging area.